In a scanning electron microscope or similar equipment, electron beams focused on the specimen must generally be circular. Depending on the condition of the objective lens and the course through which the electron beams pass, especially the stain on the aperture inserted in the objective lens, however, the focused beams form an elliptical point instead of a circular one.
This distorted condition is known as the astigmatism of electron beams.
Conventionally, such astigmatism has been corrected by provding an astigmatism correction coil d in addition to a deflection coil c which two-dimensionally scans a specimen a with electron beams b, as shown in FIG. 1. This correction is accomplished by first adjusting the focus of the specimen image to the correct focus watching the image on the screen of a Braun tube e, then controlling the current flowing to the coil d turning potentiometers f and g until the blurred specimen image becomes sharp.
In FIG. 1, reference characters h and i designate circuits generating sawtooth waves for horizontal and vertical scanning, j and k magnifying power switching circuits, l and m power sources for correcting astigmatism in the X and Y directions, and n an amplifier.
The conventional astigmatism correcting means shown in FIG. 1, however, cannot insure accurate correction, since its correction is based on the finding of a point where the specimen image on the Braun tube e becomes sharpest, turning the potentiometers f and g at the operator's discretion. This calls for a skilled operator. Then, even the correction done by the skilled operator involve a subjective process of finding out the sharpest specimen imgage.
This invention is intended to solve such problems. The object of this invention is to provide an accurate, easy-to-operate method and apparatus for correcting astigmatism in scanning electron microscopes and similar equipment.